Welcome to HCM.
You have not mentioned your age.
Since ECG is within normal limits, we have to consider other causes. It could be due to nerve
compression in the neck region ( Cervical vertebrae
), Neural pain due to inflammation of one of the nerves supplying the region, inflammation in the arm, muscular pain
caused by over exertion of muscles or due to posture adopted during sleep, peripheral vascular disease
etc. If it is simple muscular pain, painkillers could be used along with local application of muscle relaxants and analgesics. Consult a doctor and let him examine you and try to find the cause for this pain.
Wish you all the best.